feat: delete GitHub App installation when disabling integration (#4496)

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## Summary

When a user disables the GitHub integration via the Settings UI, the
system now attempts to delete the installation on GitHub's side via
`DELETE /app/installations/{id}` before clearing local state.

**Error handling**: Non-retryable 4xx errors from GitHub (404 already
gone, 403 no permission) are logged and ignored. Retryable errors -- 429
(rate limit), 5xx, and network failures -- block the operation and
return `CodeUnavailable` with a fixed customer-facing message to the
caller, preventing orphaned installations during transient outages. When
the GitHub App PEM is not configured (tests, unconfigured servers), the
GitHub API call is skipped gracefully.

Also removes the dead `DeleteGithubAppProfileAndGithubInstallationID`
RPC from the configuration service (zero callers outside its own tests;
confirmed no out-of-repo consumers).

## Changes

- **`services/api-server/service/githubapp/githubapp.go`** -- Added
`DeleteInstallation` helper using `Apps.DeleteInstallation` from
go-github/v84; non-retryable 4xx ignored, 429/5xx/network errors
propagated
- **`services/api-server/service/managementservice.go`** -- Updated
`UnsetGithubInstallationID` to call `DeleteInstallation` before clearing
local DB state; skips when PEM not configured; returns fixed error
message on failure
- **`services/api-server/service/githubapp/githubapp_test.go`** -- Added
unit tests for `DeleteInstallation` covering 204, 404, 403, 400, 429,
500, 502, 503, and network error cases
- **`sdp/config.proto`** -- Removed
`DeleteGithubAppProfileAndGithubInstallationID` RPC and message types
- **`services/api-server/service/configservice.go`** -- Removed dead
handler
- **`services/api-server/service/configservice_test.go`** -- Removed
dead tests
- Regenerated protobuf code (Go + TypeScript)

## Testing

- All `DeleteInstallation` unit tests pass (9 test cases)
- `go build ./...` passes across entire workspace
- `go vet ./services/api-server/...` clean
- TypeScript type check (`tsc --noEmit`) passes for sdp-js
- Test binary compilation for `services/api-server/service/` succeeds

## Review feedback addressed

**From `/review-changes`:**
- **Blocking (429 rate limit)**: Fixed -- `429 Too Many Requests` is now
treated as a retryable error (fails hard, like 5xx), not silently
ignored.
- **Warning (handler test gap)**: Acknowledged -- the `CodeUnavailable`
error path is covered by function-level unit tests on
`deleteInstallation`; a full handler-level test would require
NATS/Postgres mock infrastructure that doesn't exist today.
- **Advisory (removed RPC)**: Confirmed zero callers in any consumer;
all generated clients are in-repo.

**From Technical Review:**
- **Warning (error message leaks)**: Fixed -- `CodeUnavailable` now
returns a fixed customer-facing message; details remain in logs and
Sentry.
- **Warning (no auth test)**: Pre-existing gap -- the `admin:write`
scope check existed before this PR; no management service RPCs have
negative auth tests today.
- **Advisory (feature flag)**: Shipping universally was an explicit plan
decision.
- **Advisory (docs)**: Optional improvement for a follow-up; no existing
docs reference the removed RPC or the disable flow.
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Co-authored-by: Cursor Agent <cursoragent@cursor.com>
GitOrigin-RevId: 85f1a2136feed112fbf44f345b879622c374706c

Author: 2 people